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The primary goal of this investigation is to study whether soft tissue homeostatic responses secondary to decrease or increase in physiological stress levels and range of motion are a change of mechanical properties or a change of mass, or both. Two experimental animal studies are presented. One is a stress and motion deprivation study by immobilization of a rabbit knee, and the other is an increase in stress and motion study by running exercise of the miniature swine. The findings are that changes in stress and motion significantly altered the tissue properties as well as mass in the case of ligaments and digital extensor tendons. Whereas, no significant changes in properties and mass were detected for the digital flexor tendons. Possible mechanisms of the difference in tissue responses to stress and motion are discussed, and nonlinear relationships between stress and tissue remodeling are suggested.

The viscoelastic properties of the canine medial collateral ligament (MCL) were investigated. Stress-strain relationships at different strain rates, long-term stress relaxation and cyclic stress-strain curves of the MCL were obtained experimentally using a bone-MCL-bone preparation. The experimental data were used in conjunction with the quasi-linear viscoelastic theory as proposed by Fung [15] to characterize the reduced relaxation function, G(t) and elastic response sigma e (epsilon) of this tissue. It was found that the quasi-linear viscoelastic theory can adequately describe the time and history-dependent rheological properties of the canine medial collateral ligament.

Mathematical analysis by finite element modeling methods was used in conjunction with laboratory bench experiments in selecting appropriate stiffness parameters for internal fixation plate designs. Bench experiments performed included tests of an idealized plated magnesium tube and of a plated canine femur. Finite element modeling of the plate-tube and plate-bone structures was also performed, and the computed results were compared with those obtained experimentally. Three types of internal fixation plates--one "rigid" control plate and two less rigid experimental plates--were examined using both the finite element and experimental models. These plates were further examined by finite element modeling of a plated human femur during single stance loading. In all cases, the "rigid" control plate was found to shield the underlying bone from stress, while both experimental plates were found to have significantly less bone stress shielding. Our findings suggest that an improved plate design should have a low axial stiffness but moderate bending and torsional stiffnesses to facilitate fracture healing and bone remodeling without causing osteopenia.

The reconstructed anterior cruciate ligament was studied in the rabbit using the medial third of the patellar tendon. Tritiated proline, 100 microCi/kg body weight, was injected intra-articularly to insure detection of the metabolic conversion product 3H-hydroxyproline in the avascular graft. During the immediate postoperative period, nutrients were found to derive from the synovial fluid through a process of diffusion, demonstrating that synovial nutrition occurs prior to revascularization of the graft.

The effects of intermediate-term immobilization on the medial collateral ligament (MCL) of the rabbit knee are profound. Immobilization causes atrophy with reduction of collagen mass and increased collagen degradation in th immobilized MCL. The matrix composition, as reflected by collagen turnover, changed dramatically as the period of immobilization was increased from nine to 12 weeks. Collagen degradation during these three additional weeks increases, as demonstrated by the fact that at 12 weeks, collagen degradation is 28% higher than that in controls, compared with 14% at nine weeks. Degradation, balanced by new collagen synthesis, produced very minor net decrease in total collagen mass (-2%) after nine weeks, but a large decrease in total collagen mass (-27%) is observed at the end of 12 weeks. Exponential degradation has occurred as immobilization has continued, and ligament regenerative capacity apparently has failed.